Polymeric binders in conjunction with dispersing agents have been used to coat magnetic particles on films such as polyester films and disks to form magnetic recording media. High levels of the dispersing agent are usually necessary due to inefficient coupling of the dispersing agent and magnetic pigment. Drawbacks to the use of high levels of dispersing agent in conjunction with the binder include bleeding of the dispersing agent and acceleration of the hydrolytic degradation of the binder, which results in mechanical property degradation.
To overcome many of these drawbacks, various binders which themselves are dispersing agents have come increasingly into use. Several patents disclose a variety of such binders.
U.S. Pat. No. 3,490,945 ("Slovinsky") discloses a polyurethane binder for magnetic recording media which itself is a dispersing agent. The Slovinsky polyurethane binder is a moisture cured diisocyanate terminated prepolymer containing tertiary or quaternary nitrogen.
U.S. Pat. No. 4,286,022 ("Vermillion") is directed to a solution polymerized, hydroxyl terminated polyurethane block copolymer. The Vermillion polyurethane block copolymer comprises polyester or polyether segments, short chain aliphatic segments, 10-100 mole percent which contain tertiary and/or quaternary nitrogen, and symmetrical unbranched aromatic urethane segments interconnecting the polyester or polyether and short-chain aliphatic segments.
U.S. Pat. No. 5,089,344 discloses a magnetic recording medium formed on a base comprising a magnetic powder and a binder. The binder consists of a reaction product of diol, a chain extender such as 1,4-cyclohexanedimethanol, a diisocyanate and a tertiary amine.
U.S. Pat. No. 5,124,424 also discloses a binder for magnetic recording medium. The binder comprises a hydroxy-group terminated polyurethane resin prepared by reaction of a polycarbonate polyol, a chain extender, organic diisocyanate, and an active hydrogen compound having one or more hydrophilic polar groups. The hydrophilic polar amino group may be a tertiary amino group.
A dispersing binder having good dispersing characteristics, solubility, good mechanical properties and hydrolytic stability is desired.